1. Field of the Invention
The present invention relates to a semiconductor device with a capacitor, particularly a semiconductor device with a cylindrical type of a capacitor, and a manufacturing method thereof.
Priority is claimed on Japanese Patent Application No. 2007-126538, filed on May 11, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
Generally, a cell of dynamic random access memory (hereinafter, referred to as DRAM) is composed of a transistor and a capacitor. The capacitor is composed of a lower electrode (first electrode), a dielectric film and an upper electrode (second electrode). As the size of the DRAM gets smaller, the space occupied by the cell also becomes smaller. In order to acquire a certain amount of capacitance in the limited occupied-space, it is required to make an electrode structure of the capacitor to be three-dimensional, thereby increasing a superficial area of the electrode. For these reasons, a constitution has started to be used wherein the lower electrode in the capacitor is constituted by aligning a plurality of columnar portions, and then the dielectric film and the upper electrode is formed by covering the columnar portions, thereby increasing the superficial area of the capacitor and increasing the capacitance of the capacitor.
Furthermore, a constitution has also started to be used wherein the dielectric film and the upper electrode are formed inside the columnar portions, by providing a pore portions inside the columnar portions, thereby increasing the capacitance of the capacitor.
FIGS. 11A and 11B are drawings which show an example of the lower electrode in the capacitor. FIG. 11A is a schematic planar view thereof, and FIG. 11B is a schematic cross-sectional view taken along the line B to B′ in FIG. 11A.
As is clear from FIG. 11A, a lower electrode 512 is composed of the columnar portion 512a and the bottom portion 512b. Also, there is a plurality of the lower electrodes 512. Also, as is clear from FIG. 11B, the columnar portion 512a is in a tubular shape and contains the pore portion 540a inside.
When the aspect ratio of “the height in columnar direction” relative to “the diameter of the lower electrode 512” is enhanced, that is, when the diameter of the lower electrode 512 is reduced whereas the height in columnar direction is raised, a plurality of the columns can be formed in the occupied-space limited in the cell. Therefore, the superficial area of the capacitor can be increased, and the capacitance of the capacitor can also be increased.
Furthermore, the thinner the film thickness of materials which constitute the lower electrode 512 is, the wider a pore portion 540a is. Therefore, the superficial area of the capacitor can be increased, and the capacitance of the capacitor can also be increased.
As shown in FIG. 11B, a storage node contact plug (also sometimes referred to conductor plug) 502 is buried in an insulating film 501 composed of a SiO2 film which is provided on the semiconductor substrate (the substrate is abbreviated in the figure). The lower electrode 512 is formed by contacting the storage node contact plug with the bottom portion 512b, and the insulating film 503 composed of a SiN film is formed around the contacted part.
FIGS. 12A to 12C are a process drawing which shows an example of manufacturing process of the lower electrode 512.
Firstly, as shown in FIG. 12A, a storage node contact plug 502 which is composed of a conductor is formed in an oxide film 501, then a nitride film 503 and an oxide film 504 is formed sequentially, a pore portion which is the basis of the lower electrode 512 is opened, and then the lower electrode 512 is formed. Subsequently, a photoresist 513 is applied, exposed, and then developed, thereby protecting only a necessary portion of the lower electrode 512.
Then, as shown in FIG. 12B, the lower electrode 512 is etched back by conducting dry-etching, and is separated from adjacent other lower electrodes 512 (they are abbreviated in the figure). Subsequently, the photoresist 513 is removed by conducting ashing or the like.
Furthermore, as shown in FIG. 12C, an oxide film 504 used as a mold of the lower electrode 512 in FIG. 12B is removed by hydrofluoric acid treatment, thereby forming the lower electrode 512.
FIG. 13 is a schematic cross-sectional view of a capacitor using the lower electrode 512. The capacitor is formed by forming a dielectric film 514 between the tubular-shaped lower electrodes 512, and further by forming an upper electrode 515.
As a result, the lower electrode 512 has conduction only with the storage node contact plug 502 provided at the bottom side, and electric charge can be stored in the dielectric film 514 between the upper electrode 515 and the lower electrode 512.
However, in the case of forming a capacitor by using the above constitution and increasing the superficial area, there are problems that the columnar-shaped lower electrode 512 may lack physical stability because a high aspect ratio is needed, and thus the columnar lower electrode 512 may fall down in the middle of the manufacturing process.
Furthermore, in the case of increasing the superficial area by making the lower electrode 512 in a tubular shape, it is necessary to thin the film thickness of the tubular-shaped columnar portion 512a, therefore the columnar lower electrode 512 may lack physical stability, and may fall down in the middle of the manufacturing process.
FIGS. 14A and 14B are drawings which show that a part of the lower electrodes fall down, and is contacted with adjacent lower electrodes 512. FIG. 14A is a schematic planar view thereof, and FIG. 14B is a schematic cross-sectional view taken along the line C to C′ in FIG. 14A. Letters and numerals in FIGS. 14A and 14B are shown in the same way as in FIGS. 11A and 11B.
When a capacitor is formed in such a state, an electrical short circuit may be caused, therefore it is a big problem as a semiconductor device.
Some approaches have been suggested, taking the above circumstances into consideration. For example, in Japanese Unexamined Patent Application, First Publication, No. 2003-142605, there is disclosed a constitution in which a support called as “insulating beam” is formed between the lower electrodes, thereby preventing the lower electrode from falling down. Also, in Japanese Unexamined Patent Application, First Publication No. 2003-297952, there is disclosed a constitution in which a support called as “frame” is formed between the lower electrodes, thereby preventing the lower electrode from falling down. However, in methods described in the above references, straight-line insulating beams are provided in a horizontal or vertical direction, relative to the lower electrode aligned regularly in rows and columns. Alternatively, straight-line insulating beams are provided both in a horizontal direction and in a vertical direction, thereby preventing the lower electrode from falling down. Therefore, in a layout shown in FIGS. 11A and 11B in which the lower electrodes are aligned densely, it is required that the insulating beam with a narrower width than the width of the resolution limit of lithography should be formed in a oblique direction, so it is difficult to be achieved.
On the other hand, FIGS. 15A and 15B show a conventional example using an insulating beam with a width capable of being formed in lithography. FIGS. 15A and 15B are an example showing an example of a capacitor in which a beam 605 is formed. FIG. 15A is a schematic planar view thereof, and FIG. 15B is a schematic cross-sectional view taken along the line D to D′ in FIG. 15A. In the capacitor, the constitution of the lower electrode 612 is the same as the constitution of the lower electrode 512 shown in FIGS. 11A, 11B, 12A and 12B, except that the beam 605 is formed in FIGS. 15A and 15B, but not formed in FIGS. 11A, 11B, 12A and 12B.
As is clear from FIG. 15A, the beam 605 is formed in a line, contacting the side surface of a plurality of the lower electrodes 612. In this case, the beam 605 is formed in a line with a large width which can be formed by lithography, therefore the difficulty to form the beam itself can be reduced.
Furthermore, as is clear from FIG. 15B, the beam 605 is formed between adjacent lower electrodes 612, whose upper side are contacted by the beam 605. Therefore, the capacitor can prevent the lower electrode 612 from falling down. Here, the beam 605 is formed by a SiN film.
However, even if the beam 605 is used, there are such problems that the beam 605 may be removed from the lower electrode 612, and may be bent, and further the lower electrode 612 may fall down due to a wet treatment (washing step) in the manufacturing process of a semiconductor device, or due to a surface tension taken in the drying step accompanied with the wet treatment.
FIGS. 16A and 16B are drawings showing that the lower electrode 612 in the capacitor shown in FIGS. 15A and 15B falls down, due to the washing step. FIG. 16A is a schematic plan view thereof, and FIG. 16B is a schematic cross-sectional view taken along the line E to E′ in FIG. 16A. Letters and numerals of members shown in FIGS. 16A and 16B are the same as those in FIGS. 15A and 15B. It is found that three out of a plurality of the lower electrodes 612 are removed from the beam 605, and fall to adjacent lower electrodes 612
As a technique to avoid such a problem that the lower electrode is removed from the beam as described above, it is considered to increase the film thickness of the beam 605. The thicker the film thickness is, the larger the contacting portion can be, thereby strengthening the resistance against collapse of the lower electrode 612. Therefore, the inventors tried to form the beam 605 with a film thickness of about 100 nm to 150 nm.
FIGS. 17A to 17D and FIGS. 18A to 18D are process drawings, each of which shows an example of manufacturing steps to form a lower electrode 612, in which the film thickness of a beam 605 is thickened.
FIG. 17A is a cross-sectional view showing the portion where the lower electrode is formed in the etching process of the beam 605. The etching surface 605a of the insulating film 605 is formed with a slope in the direction of the film thickness.
The steps shown in FIGS. 17B to 17D and FIGS. 18A to 18B are conducted, using the beam 605. In the case of forming the lower electrode 612, a pore portion 640a of the lower electrode 612 is tapered at the portion where the beam 605 is formed, and it is formed with a narrower space at the region below the beam 605, as shown in FIG. 18C.
In this way, a pore portion 640a with a narrower space decreases the capacitance of the capacitor. Therefore, it becomes a factor causing the deterioration of the characteristics of the capacitor.
In the case of further increasing the film thickness of the beam 605, the pore diameter becomes smaller at the bottom part of the lower electrode 612, therefore sometimes the pore cannot be achieved.
Moreover, in the case that exposure output is increased in order to etch the beam 605 forcedly in a vertical form, a photoresist 606 used as a mask cannot endure the exposure output. As a result, a pore diameter is expanded, the adjacent lower electrodes 612 get close to each other, and thus the capacitor itself becomes a structure to have a tendency to short out.
Furthermore, in the case of increasing the film thickness of the beam 605, parasitic capacitance between the lower electrodes 612 of the capacitor is increased because a nitride film has a relative permittivity of about 7. As a result, there is a problem such that a high-speed operation of a semiconductor device is interrupted. Particularly, it becomes a big problem in manufacturing a device capable of carrying out a high-speed operation wherein a trace of 70 nm or less is used. From the above results, it is clarified that a required semiconductor device cannot be realized only by increasing the film thickness of the beam.
Taking the above into consideration, a semiconductor device and a manufacturing method thereof are desired, in which a beam 605 is not easily removed from a lower electrode 612, in spite of the fact that the film thickness of the beam 605 is as thin as possible in order to decrease parasitic capacitance between the lower electrodes 612 thereby enabling a semiconductor device to carry out high-speed operations.